Hydrocarbons in the form of oil are found in reservoirs in the earth. The oil is recovered by penetrating the reservoirs with well bores. Several stages may be used to produce the oil found in underground reservoirs. The first stage, which is known as the primary production stage, allows the oil to flow into the well bore under natural forces. At first, the natural forces in the reservoir may be sufficient to drive the oil to the surface where it is recovered. However, at some point, pumps may be required to displace the oil from the well bore to the surface. The primary production stage usually yields only about 5% to 15% of the oil in the reservoir.
A secondary recovery operation thus is commonly performed to recover additional amounts of the oil from the reservoir. A common secondary recovery operation known as secondary flooding involves injecting a fluid such as water into a so-called injection well (or wells) to drive oil in the reservoir to a second well (or set of wells) known as the producing well. Secondary flooding usually recovers up to an additional 50% of the original oil in the reservoir. However, a large portion of the original oil often remains in the reservoir even after secondary flooding. As such, tertiary recovery operations have been developed to increase the amount of oil recovered from the reservoir. One common tertiary recovery operation, which is known as tertiary flooding, employs yet another fluid, e.g., a fluid that is miscible or partially miscible with the reservoir oil, to drive the oil from the reservoir to the producing well.
As is known in the art, oil reservoirs often contain fractures, vugs, voids, fissures, and high permeability streaks that form so-called least resistant flow paths (or permeable zones) between the injection well and the production well. During secondary or tertiary flooding the injectant, i.e., the fluid injected into the injection well to drive the oil to the production well, tends to pass through the more permeable zones, bypassing the less permeable zones and thus leaving significant amounts of oil in the reservoir. In addition, the pressure drop across the reservoir required to enhance the displacement of oil from the reservoir often cannot be achieved due to the presence of the more permeable zones. The amount of oil displaced by the injectant is thus reduced, resulting in a reduction in the sweep efficiency of the injectant. As the injectant continues to pass through the more permeable zones, the rock structure within those zones erodes, thus exasperating the problem. Moreover, the permeable zones may allow the injectant to prematurely communicate with the production well, resulting in the recovery of the injectant rather than the oil. Therefore, the fractures, vugs, voids, fissures, and high permeability streaks in the reservoir undesirably limit the amount of oil that can be produced.
A need therefore exists to develop a method for blocking the permeable zones in an oil reservoir to thereby improve the sweep efficiency of an injectant used during recovery operations. Blocking the permeable zones would also allow a desired pressure drop across the reservoir to be attained. As a result, oil production could be increased despite the presence of fractures, vugs, voids, fissures, and high-permeability streaks in the reservoir.